1. Field of the Invention
The present invention relates to an image display unit capable of displaying a color image with a high resolution and in particular to a display control circuit which is suitable for writing data into a display memory in dot-by-dot coloring.
2. Description of the Prior Art
As an apparatus for reading out data written in a display memory of a personal computer, for example, and graphically displaying the data on a display screen such as a CRT, higher resolution of color display is required and it is necessary to color picture cells dot by dot. Thus, a display memory having a very large capacity is necessitated. Therefore, writing data into the display memory by using only software often results in an insufficient display speed. In the so-called bit map system which is most widely used at the present time, one bit of data corresponds to one dot on the display screen. For writing only one dot with a designated color into the memory in such a bit map system, the following steps must be executed for each of a plurality of display memories.
(1) The color data already stored at an address at which new data is to be stored is read into a CPU register.
(2) The color data thus read out is ORed with the data to be written.
(3) The resultant ORed data is stored in the address.
Such processing which requires a long time, provided that software is used alone, can be executed in a short time by adding a suitable circuit. Such an additional circuit is described in U.S. application Ser. No. 488757 previously filed by the inventors of the present application, for example.
The above described bit map system will now be described by referring to the drawings. FIG. 1 is a block diagram of an example of a prior art display memory unit using the above described bit map system. In FIG. 1, numeral 1 denotes a CPU (Central Processing Unit), numeral 2 denotes an address bus, numeral 3 denotes a data bus, numeral 4 denotes a read/write selection signal line, and numeral 5 denotes an address decoder. Numerals 6, 7 and 8 denote display memories for three principal colors, i.e., red, green and blue (hereafter abbreviated as R, G and B, respectively). Numeral 9 denotes a display memory for intensity (hereafter abbreviated as I). A data gate circuit 10 constitutes a memory control means and gates data from the data bus 3 under the control of a display memory selection signal 11 fed from the address decoder 5. A data bit selection signal 12 fed from the data gate circuit 10 selects respective memory chips of each of the display memories 6, 7, 8 and 9. Color registers 13, 14, 15 and 16 constitute data supply means for directing write data into the display memories 6, 7, 8 and 9, respectively. Output signals of these color registers are fed to data input terminals of the respective associated display memories.
In the display memory unit shown in FIG. 1, the data gate circuit 10 selects one or more memory chips in each display memory for each data bit to be written into the display memories 6, 7, 8 and 9. It gates the data on the data bus 3 in response to the display memory selection signal 11 and supplies the output signal to the display memories 6, 7, 8 and 9 as the bit selection signal for the respective memory chips of the display memories. The color registers 13, 14, 15 and 16 store color information of the graphic patterns for instructing whether or not the graphic data to be displayed is to be written into the display memories 6, 7, 8 and 9 for three principal colors and intensity. Since one bit is stored in each of the color registers 13, 14, 15 and 16, four bits in total are stored in these registers. The color information is written into the color registers 13, 14, and 15 from the data bus. That is to say, color codes corresponding to R, G, and B may be directly written into the color registers 13, 14 and 15, respectively.
FIG. 2 shows a circuit diagram to illustrate the connection of the display memory 6, the data gate circuit 10 and the color register 13 as shown in FIG. 1. The peripheral circuits of the display memories 7, 8 and 9 are also connected in the same manner. In FIG. 2, the data gate circuit 10 comprises eight AND circuits and it supplies the data on the data bus 3 to the memory chips of the display memories 6, 7, 8 and 9 as memory chip selection signal 12 at the time of receipt of the display memory selection signal 11 from the output of the address decoder 5. The color register is a one-bit latch and the output signal thereof is supplied to the display memory 6 as the data entry signal to the display memory 6.
Four display memories 6, 7, 8 and 9 are provided. Each of the four display memories comprises as many memory chips as data lines on the data bus 3. That is to say, only one bit of data per address can be stored in one memory chip.
The writing of the data into the display memory in the display memory unit shown in FIGS. 1 and 2 is now explained. The CPU 1 designates the colors of the graphic pattern to be displayed by four bits, one for each of R, G, B and I, and the color information is stored in the color registers 13, 14, 15 and 16.
The writing of the data into the color registers 13, 14, 15 and 16 is performed via the data bus 3. In one example, bit 0 on the data bus 3 corresponds to R, and bits 1, 2 and 3 correspond to G, B and I, respectively.
The brightness information for controlling whether or not new color information is written to be displayed is then written at the address of each of the display memories 6, 7, 8 and 9 corresponding to the address on the display screen. The address of each of the display memories 6, 7, 8 and 9 is provided on the address bus 2 and the address decoder 5 produces the display memory selection signal 11 upon detection of an address on bus 2 which falls within the range of addresses allotted to the display memories 6 to 9; so that the data gate circuit 10 is opened. On the other hand, the brightness information to be written into the display memory is provided on the data bus 3, and this data is supplied to the display memories 6, 7, 8 and 9 through the data gate circuit 10 as the memory chip selection signal 12.
The signal 12 controls the selective writing of data into each chip of the display memory 6. Since the chip selection signal 12 is obtained by inverting the pattern data, only the bits having pattern data "1" are selected to be written, and the bits having pattern data "0" are not selected to be written. As a result, the writing of the color information of the color register 13 is permitted for the chip of the display memory to which the "1" brightness information data is supplied, and the writing of the data is not permitted for the chip of the display memory to which the "0" data is supplied. That is to say, the writing of data is controlled depending upon whether the brightness information data is "1" or not. Accordingly, when the new data is overwritten on the recorded data at the same address of the display memory, the software processing of writing the ORed data of the recorded data and the new data as is done in the prior art unit is not necessary, but only the data to be newly written need to be written into the display memory. The color data stored in the above described color register is written only for the data bits newly written, that is, the bits to which "1" information are supplied, and the data remains unchanged for those bits which have been previously recorded. Therefore, the bit map system raises the display speed. As evident from FIG. 2, however, it must be possible to control the enable/disable condition of each bit of the display memory in the bit map system. Accordingly, memory chips having one bit per address had to be used in parallel. In this case, the number of required elements is usually equal to the number of the data bus lines of the CPU (number of bits of data outputted by the CPU); multiplied by the number of colors, i.e., the number of planes in the depth direction of the display screen. For example, in a system having an 8-bit CPU and the above described display memories for R, G, B and I, 32 (=8.times.4) memory chips for the display memories are required. For a display screen having 640 dots in the horizontal direction and 200 dots in the vertical direction, which is the most common display screen in th current personal computer, the necessary total capacity of display memories for R, G, B and I is 512,000 (=640.times.200.times.4) bits. This capacity can be obtained by using eight 64-kbit dynamic RAM's as the memory chips for each display memory;. However, in the disclosed system 32 memory chips must be used because each bit of the display memory must be controlled to be in the enable/disable condition. This results in problems including the increased cost, enlarged board size, and increased unnecessary area on the display memory.